The user capacity of mobile radio communication systems is limited by the width of the frequency spectrum available for signal transmission. In order to maximize a system's capacity, therefore, it is desirable to utilize the available frequency band in the most efficient manner possible.
Cellular telephone systems in operation today commonly use an access technique known as Frequency Division Multiple Access (FDMA) to permit a base station to communicate with a plurality of mobile stations. In FDMA systems, each communication link is allocated a unique frequency slot of channel in the radio spectrum.
Newer systems use Time Division Multiple Access (TDMA), in which a base station communicates with a plurality of mobiles on the same frequency channel by dividing up a time cycle into time slots. The European GSM standard is an example of a system using FDMA and TDMA to allocate both frequency and time slots to mobile calls. The system uses 200 KHz wide frequency slots in each of which a 4.6 mS transmission cycle is divided into eight, 560 uS time slots, with short guard periods between each.
The guard periods in GSM are provided because base station transmission during a time cycle is not held at a constant power for all time slots, but instead changes the power level for each time slot based on the distance of the mobile station using that time slot from the base station. Moreover, for transmissions which employ frequency hopping, wherein the frequency channel employed for each 4.6 mS time cycle changes, a guard period of zero transmission power is provided whenever power or frequency is changed discontinuously to avoid spectral splatter into other frequency channels.
Another example of a system employing both TDMA and FDMA is the US Telecommunications Industry Association standard IS54. The IS54 standard describes a system having 30 KHz wide time slots, in each of which a base station employs a 20 mS transmission cycle divided into three, 6.6 mS time slots with no guard period between. The base station transmission in this system is actually just a continuous transmission of time-multiplexed data to three mobile stations. There is no guard period provided in TIA IS54 because frequency hopping is not employed, on the contrary, the system anticipates that the power level will be the same in all time slots.
U.S. Pat. No. 4,866,710 to Schaeffer describes a method of allocating frequencies and time slots to mobile stations such that all the time slots on a given frequency are filled first before allocating time slots on another frequency. By packing mobile stations preferentially in this way, the transmitters and frequencies that have not as yet allocated time slots can be switched off completely, reducing interference. This would reduce wasted capacity in the IS54 system arising from the requirement that base stations continually transmit on all three time slots even when only one is needed. However, it will be noted that the base station still transmits at one maximum power level for each frequency in use, irrespective of the power needs of each particular mobile, resulting in a higher net level of interference than if the power needs of each mobile were taken into account.